Another UN-run school in Gaza has reportedly been struck by an Israeli air strike, killing at least 10 people who had been sheltering there from the fighting.

The UN Secretary-General Ban Ki-moon condemned the attack as "a moral outrage and a criminal act". The director of UN's Palestinian refugee agency in Gaza said preliminary findings indicated an Israeli missile was to blame.

Witnesses and health workers said around 35 more were wounded when the entrance to the facility was struck by. It is the second reported strike on a UN-operated school in less than a week.

Hundreds had been sheltering in the building in the southern Gaza Strip town of Rafah, where the death toll rose to more than 100 in 24 hours yesterday.

The Israeli military confirmed today that it had scaled down some of its military presence in northern Gaza and told people they could return to their homes, but insisted the offensive would continue until rockets stopped firing and Hamas's network of cross-border tunnels had been destroyed.

It offered no immediate comment about the Rafah school bombing, but confirmed that officials were investigating the incident.

In a statement, Mr Ban said the strike was "yet another gross violation of international humanitarian law".

"This attack, along with other breaches of international law, must be swiftly investigated and those responsible held accountable," the UN chief said. "It is a moral outrage and a criminal act."

Today’s attack came as the British Foreign Secretary, Philip Hammond, said the conflict had become "intolerable".

Last Wednesday, at least 15 Palestinians at a UN school in the Jabalia refugee camp were killed during fighting, with UN observers saying it appeared an Israeli artillery shell had hit the building.

Then, the military said gunmen had fired from mortars placed near the school and that the army had shot back in response.

Palestinians aid injured people on the ground following an Israeli military strike on a UN school in Rafah, in the southern Gaza Strip (AFP)
Speaking after the Labour party leader Ed Miliband accused David Cameron of getting it "wrong2 with Britain’s position on the Gaza conflict, Mr Hammond told The Sunday Telegraph: "The British public has a strong sense that the situation in Gaza is simply intolerable and must be addressed - and we agree with them.

"There must be a humanitarian ceasefire that is without conditions. We have to get the killing to stop."

Downing Street accused Mr Miliband of "playing politics" and insisted the Prime Minister had always been clear that both sides in the conflict should observe a ceasefire.

The row began with the Deputy Prime Minister, Nick Clegg, after he warned that the Israeli assault on Gaza appeared "disproportionate" in response to Hamas’s rocket attacks.

Israel has now lost 64 soldiers and three civilians in the conflict, its highest death toll since the 2006 war in Lebanon.

The UN said that at least six of its facilities, some housing Palestinians already displaced by the fighting, have been struck by Israeli fire since the Israeli offensive began on 8 July. In each previous case the Israeli military has said it was returning fire towards militants set up near the compounds.

The UK's Department for International Development said that, in total, 136 schools, 24 hospitals and clinics and 25 ambulances have been damaged or destroyed, while eight UN aid workers and at least two Palestinian Red Crescent volunteers have now been reported as killed.

Local officials in Gaza say more than 1,750 Palestinians have now been killed in the conflict, the majority of them civilians, while more than 8,000 people have been wounded.